A conventional capacitor using a dielectric having a dielectric constant higher than that of a silicon oxide film or a ferroelectric having a dielectric constant higher than that of a silicon nitride film has a large capacitance per unit area. Applications require large capacitance with small area, particularly applications such as large-scaled DRAMs have been being examined. In such structures, the dielectric having a high dielectric constant is defined as a material having a dielectric constant higher than that of the silicon oxide film. Ferroelectric materials have a spontaneous polarization which can be inverted by an electric field. In particular, as ferroelectrics, complex-metal oxides such as (Ba, Sr)TiO3 (hereinbelow, BST) and Pb(Zr, Ti)O3 (hereinbelow, PZT) are being examined. In order to suppress degradation of the oxides upon film formation, a noble metal such as platinum having oxidation resistance is typically used as a lower electrode. Meanwhile, since an upper electrode is generally formed after film formation, in order to avoid reaction with the dielectric having a high dielectric constant in a heat treatment process after formation of a capacitor, the upper electrode is generally made of platinum.
For example, according to U.S. Pat. No. 5,005,102, a lower electrode has a structure of platinum/titanium nitride/titanium and an upper electrode has a structure of aluminum/titanium/platinum. In particular, with respect to the upper electrode, it is described that aluminum serves as an electrical contact layer, titanium serves as a diffusion barrier layer, and platinum serves as a plate layer.
In the case of fabricating a memory using those elements, after forming a capacitor, a wiring layer for electrically connecting to the capacitor and a wiring layer related to a peripheral circuit part for performing electrical conversion between a memory cell and the outside of the memory chip are formed. In order to obtain electrical insulation between the wiring layers and between the wiring layers and the capacitor, it is necessary to form interlayer insulating films. This process is performed in a reducing or weak acidic atmosphere in order to prevent degradation of the wiring layers. Since a through hole for electrically connecting the peripheral circuit and the wiring layers generally has a shape with a high aspect ratio, which is deep as compared with the size of the opening, tungsten or the like is deposited by a CVD process. The atmosphere at this time is a reducing one. It is known that the capacitor is seriously damaged by being subjected to the treatment in the reducing atmosphere. For example, according to “Material Research Society Symposium Proceedings”, Vol. 310, pp. 151 to 156, 1993, it is reported that by forming an SiO2 film by a CVD process, PZT as a ferroelectric loses its ferroelectricity characteristics and a leakage current increases.
Further, although the characteristics of a semiconductor active device degrade due to a heat treatment in a capacitor fabricating process, a plasma process in a wiring process, and the like, by additionally performing a heat treatment in hydrogen at approximately 400 degrees after completion of the wiring process, the degradation can subsequently be repaired. Consequently, a hydrogen treatment is generally performed after completion of the wiring process. It is known, however, that the hydrogen treatment exerts an influence on the characteristics of the capacitor in a manner similar to the interlayer insulating film process. For instance, according to “8th International Symposium on Integrated Ferroelectrics, 11c”, 1996, it is reported that, in the case where SrBi2Ta2O9 (hereinbelow, SBT) is used as a ferroelectric, the capacitor is peeled off or, when the capacitor is not peeled off, a leakage current characteristic largely deteriorates.
The dielectric having a high dielectric constant and the ferroelectric will be generically called a high dielectric constant or ferroelectric film hereinbelow.
It is an object of the invention to obtain a very reliable semiconductor storage device in which the high dielectric constant or ferroelectric film is prevented from degrading.